Dizziness and nausea are the major complaints leading to consultations in medical clinics or emergency units. Though many systemic problems can be associated with these symptoms, the majority are caused by deficits in the peripheral vestibular apparatus, or its associated projections in the brainstem and cerebellum. Despite this high clinical profile, there has been little change in the testing protocols, and large, expensive, and necessarily stationary equipment in hospital environments are required under current protocols.
It is generally assumed that the vestibulo-ocular reflex (VOR), for example, can be probed simply by comparing responses in the dark to those in the light with room- or body-fixed targets. The differences are normally ascribed to the visual system. These approaches are not technically correct, since the visual and vestibular processes in the brain share common pathways. Hence, we are actually dealing with a two-input system at all times (whether targets are imagined or provided).
Modern vestibular tests applied in a hospital otolaryngology clinic include:
bedside qualitative observations of spontaneous or elicited nystagmus (e.g. by head-shaking); PA1 caloric irrigation (hold or cold) of the ear canal to probe asymmetries between right and left systems; and PA1 passive whole body rotation of a patient with selected speeds and profile, again to probe symmetry, linearity, and their modulation by interactions with visual goals (e.g. VOR dark vs. light VOR suppression while viewing head-fixed target). PA1 a. Displaying to the subject at least one target undergoing slow random motion; PA1 b. perturbing the head of the subject with perturbations statistically uncorrelated with the random motion of the at least one target; PA1 c. measuring at least one dimension of motion of the head of the subject; PA1 d. measuring a dimension of motion of at least one eye of the subject; and PA1 e. estimating visual and vestibular system response dynamics based on the measured head and eye responses to dual perturbations.
It is known that any head motion (linear or angular) will cause eye movements consisting of two mixed segments: slow-phases, where the eyes and head move in opposite directions, and fast-phases (saccades) which redirect the gaze before the eyes reach motor limits. The ensemble of slow and fast phases is referred to as ocular nystagmus. The usual measures of VOR function include the gain (eye velocity/head velocity) and phase of the slow-phase trajectories during sinusoidal rotations or linear motions. Because these tests are performed by rotating the subject's entire body, large high-torque motors are required--an expensive approach not readily applicable to home or community clinic use.
Classically, test protocols are based on the assumption that visual and vestibular systems can be assessed independently, and that VOR tests in the dark reflect the dynamics of the default system with no visual goals. In fact, dark responses can be modulated even by imagined targets and so can be very labile; furthermore, the visual and vestibular oculomotor system share most of the relevant pathways in the brainstem. As a result, the dynamics and linearity of apparent VOR responses will be sensitive to any correlation with real or imagined visual goals. This explains why the measured `VOR` gain rises to one (perfect compensation) when viewing an earth fixed target, and drops to less than 0.3 when viewing a head-fixed target. The `default` value in the dark can vary between 0.5-0.8 in normal adults, and one wonders what purpose a VOR in the dark can serve operationally, sine the ultimate goal is stabilization of a regtinal field, using both visual and vestibular sensors.
Often tests of the VOR in the dark cannot distinguish easily between normal subjects, and vestibular patients who have had time to adjust or compensate for their deficits. It is even more difficult to detect differences between those compensated patients leading normal lives, and those complaining of nausea or degradation of life style. These issues are important for separating malingerers from true patients, and for the certification of employees in high-risk jobs (e.g. airplane pilots, large vehicle controllers, high-rise construction workers, etc. It would be more appropriate to test subjects using a more normal context of visual and vestibular co-stimulation, since it is their performance when active and viewing targets, not in the dark or when their eyes are closed, that affects comfort level and functional limits.
The length of clinical rotation tests is another issue. Often, the dynamics are probed using single sinusoids, and varying amplitudes are applied to test linearity; hence such tests can be very time-consuming for a patient and have a high risk of subject malaise. Those few clinics which instead apply pseudo-random test sequences have the advantage of short test periods, but again rely on large motor platforms, and classical dark/light test conditions, which, as mentioned above, lead to correlated input conditions.